For example, with mobile telephones and Voice over Internet Protocol (VoIP), a voice signal is transmitted after the voice signal is converted to a narrowband (e.g., 300 [Hz] to 3400 [Hz]) and consequently, the voice signal deteriorates (e.g. generation of a muffled-voice sound). As a countermeasure, a technology is conventionally known of copying a frequency component of the narrowband voice signal to an expansion band, thereby pseudo converting the signal to a wideband signal. For example, a method is disclosed of generating a high band signal by copying a component of an input signal to a high band and obtaining a low band signal by full wave rectification of the input signal (see, e.g., Japanese Patent Laid-Open Publication No. H9-90992).
The conventional technology described above, however, cannot sufficiently obtain the effect of the band expansion, depending on the noise included in a received voice signal or the noise on the reproducing side. Further, voice quality could further deteriorate as a side effect of the band expansion. For this reason, there is a problem in that the conventional technology described above is incapable of sufficiently improving the quality of the voice to be reproduced.